1. Field of the Invention
This invention relates to apparatus and process for detecting a single specific chemical solid or gas in the presence of other chemical solids or gases or for detecting a number of specific mixed chemical solids or gases. The apparatus and process is particularly suitable for detecting a single specific gas in the presence of other gases or for detecting a number of specific mixed gases. The apparatus and process passes multiple beams of electromagnetic radiation incident on the material, such as gases, in a sample chamber producing acoustic signals at specific frequencies when the light of specific wavelengths is absorbed. The electromagnetic radiation is selected to have wavelengths corresponding to optical absorption features of a single specific chemical or a number of specific mixed chemicals in the sample chamber. The acoustic signals are detected by a beat tone or a combination tone characteristic of and representative of the degree of absorption by the specific chemical or multiple specific chemicals desired to be detected in the sample. The apparatus and process are useful in leak detection in natural gas systems and chemical processes and for detection of pollutants, explosive gases and the like.
2. Description of the Prior Art
The need to detect a specific chemical, such as gas, or a mixture of specific chemicals, such as gases, in a sample, such as a gas stream, frequently arises and a number of detection techniques have been devised. One frequent need for detection of a single specific gas arises in connection with location of leaks in natural gas systems, in the detection of pollutants, poison or explosive gases. Many of the gas detection techniques currently available do not provide desired specificity and are not suitable for field use. For example, currently used devices for leak survey in natural gas pipelines include combustible gas indicators and flame ionization detectors, both of which are incapable of distinguishing methane from a methane-ethane mixture, and gas chromatographs which do make such distinctions, but are not practical for field utilization.
Various sonic leak detectors have been proposed which do not detect a single specific gas, such as: U.S. Pat. No. 3,055,209 teaching an active sonic leak detector by transmitting sonic waves through the gas within a conduit and through the leak opening for detection; and U.S. Pat. No. 3,264,864 teaching transmission of a sonic wave of one frequency through a gas conduit and detecting a mechanical wave of sonic frequency transmitted by the gas through a leak in the conduit. Acoustic means have been used to detect specific gases. U.S. Pat. No. 3,429,177 teaches hydrogen gas detection by change of velocity of specific wavelength acoustic waves in passing through the hydrogen; U.S. Pat. No. 3,981,176 teaches determination of gas composition by measurement of acoustic impedance and absorption of gas at two distinct frequencies; and U.S. Pat. No. 4,246,773 which teaches determination of specific gravity of a gas by determining the sonic speed in the gas by electrical measurement of a beat frequency obtained by mixing the output of an acoustic detector wherein the sound is passed through a standard gas with the output of a detector in which the sound is passed through the gas in question. U.S. Pat. No. 3,938,365 teaches detection of trace amounts of one gaseous species in a sample chamber acoustically in response to a high intensity light source having its beam modulated at a frequency corresponding to an acoustical resonant frequency of the sample chamber. The photoelectric and acoustical signal leaving the sample chamber are compared and the acoustical signal provides an indication of energy absorbed by the gas in the sample chamber at the light wavelength. The process taught by the '365 patent allows only identification of a single gas species.
The prior art does not, to the inventors' knowledge, suggest any devices capable of detecting multiple specific gases in a mixture of specific gases nor any devices capable of detecting a specific gas in the presence of other gases which absorb the same electromagnetic wavelengths.